1. Field of the Invention
The invention relates to a method of curving an edge portion of a sheet of deformable material (e.g. metal) and to a bending tool for carrying out the method.
2. Description of the Prior Art
When shaping thick sheet metal (e.g. 20-200 mm thick) for cylindrical containers, each sheet is bent by rolling it or by successively bending it in a hydraulic press. In the latter case the sheet is supported on two spaced-apart elongated pads and is subjected to a press force from an elongated punch engaging the sheet at a location between the two pads. Using such a press arrangement or a conventional three-roll bending press, the edge portion(s) of the sheet cannot be given the desired curvature, but a plane edge portion having a width of at least twice the sheet thickness, and often more, remains. The difficulties are accentuated with increasing sheet thickness and are particularly great when the sheet thickness exceeds 70-80 mm. With known pre-bending methods for bending an edge portion, the sheet is subjected to such high specific pressure from the convex surfaces of pads or rolls, that permanent yielding of the sheet material may occur in the edge region. The risk of undesired deformation of the edge region of a sheet is particularly great during simultaneous sliding between the sheet and the outermost die bar. The drawback is particularly great if a sheet edge has been carefully machined in preparation for the forming of a weld joint.